So basically, we don’t know why Flappy Bird is no longer available, as of February 9th. But it sounds like he is tired of the attention the game has been getting, with him getting asked for interviews. He is probably also tired of the hate this game has been getting from users who complain that it is too hard.
So if you want to play Flappy Bird (and probably delete it shortly afterwards), you’d better do it while you can. Especially iOS users. Android users will probably be able to find the APK online, but iOS users will have a tougher time finding it. Also, now there will be no Windows Phone version of Flappy Bird.
So, as usual Flappy Bird is free on both the App Store and Google Play. Personally, I wish the developer best of luck with his future endeavors, along with the money he has and will continue to make from the ads.